It doesn't seem to get bigger when the oven is hot - though I'm not sure if that fact should be a comfort or something to worry about.

The vent is out of a castable refactory material that was very liquid and black when mixed, and it is supporting a rather heavy chimney. That wall of the vent is actually rather thin, with most of the weight resting on the bricks of the oven entry.

The last pic is of my arch crack, which ran all along the the concrete covering when the oven was hot. You can't see it because I painted over the top hoping that the paint will fill it up...

I don't see there's much I can do about it, and I'm not specially worried if it doesn't get bigger, but ...what does everybody think?

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